1. Field of The Invention
The present invention relates generally to a rear wheel steering system for an automotive vehicle and more specifically to an improvement in a gear train for steering rear wheels.
2. Description of The Prior Art
Japanese Utility Model First Publication No. 3-86882 discloses a gear train arrangement of a rear wheel steering system for an automotive vehicle. This gear train includes a worm gear and stopper pins. The worm gear is driven by a D.C. motor and coupled with tie rods to which rear wheels are rotatably attached. The stopper pins restrict rotational movement of a worm wheel over a preselected steering angle range. The worm gear is designed to assume a reverse rotation efficiency less than zero for restricting displacement of tie rods caused by external force acting on tile rear wheels. In operation, the system monitors a steered angle of the front wheels and vehicle speed to determine a rear wheel steering target angle according to a preselected algorithm. The system further detects an actual steered angle of the rear wheels to control rear wheel steering operation so as to compensate for the difference between tile actual steered angle and tile target angle.
The above prior art gear train of the rear wheel steering system has, however, suffered from certain drawbacks, in that when the D.C. motor overruns due to a malfunction of the system, for example, the worm wheel is urged to rotate into contact with either of the stopper pins, thereby causing excessive torque to act on the stopper pin due to the inertial torque of the motor or so forth. This results in tight engagement between a worm and the worm wheel due to a reaction force caused by elastic deformation of the stopper pin. Therefore, returning the rear wheels to a neutral position where a steered angle is zero requires a higher torque of the motor.